High temperature stainless steel



z,75s,02s f 7 HIGH TEMPERATURE srAm Ess STE L] William Charles Clarke,Jr., -Dundalk, Md., to. Armco Steel Corporation, a corporation of OhioNo Drawing. Application May 23,1952, a

Serial No.289,657 I l 3 Claims. (c1. 75-5128 1 This application forpatent is a" continuation-impart and durable high temperature austeniticstainless steel,

and products and articles of the steel, which are capable of resistingthe" development of sigma phase while under load at high temperatures,which are resistant to stress-rupture and creep under load at'thehigh'tempera- M tures encountered, and which are well suited forresisting attack by hot corrosive matter and avoiding the formation ofheat scale.

Another object of my invention is the provision of high temperaturestainless steel of thecharacter indi cated which is well protectedagainst carbide precipitation and intergranular corrosion.

A further object of the present invention is that of has good hotworking properties. Other objects of the invention in part will beobvious and in part pointed out more fully hereinafter. The inventionaccordingly consists in' the combination of elements, features of thesteel, and in the features 40 of articles, products and manufactures ofthe steel, as

described herein, the scope of the application of which is indicated inthe following claims. I

As conducive to a clearer'understanding' of certain aspects of myinvention, it may be noted at this point that stainless steel in moregeneral terms is defined as a steel which contains about 10% to "35%chromium, with or Without nickel, and sometimes, additions of suchelements as manganese, silicon, cobalt, copper, molybdenum, tungsten,vanadium, columbium, titanium, sulphur,

or the like, for special purposes; the remainder of the steel beingsubstantially all iron except for incidental impurities. Among thesesteels are the austenitic' chromium-nickel steels, which have manyfavorable properties. In this connection, though, high temperatureprodnets and articles made of any of a variety of the austenitic steelsare extremely susceptible to creep and stressrupture when under load forextended periods of time While hot. Many of-the products tend to form aconstituent at high temperatures called the sigma phase.

This phase is definitely related to the composition of the product orarticle and the high temperature use. It is hard and brittle at roomtemperature, and is a non-magnetic intermetallic compound which is weakand ductile at high temperatures. phase slowly develops and when presentin quantities above about 2%, impairs creep strength and stress-ruptureproperties. More than small quantities of the phase, therefore, arehurtful under high temperature conditions of use of the steel, if for noother reason than introducing a weakening effect with regard tostressrupture and creep.

tion required for normal corrosion resistance.

tially all iron.

While the steel is heated, this 35 2,758,025 i ear n e u Many austeniticstainless steels are susceptible to intergranular corrosion particularlyafter exposure to temperatures around and approaching 1500 F., and thussuch a carbide draws heavily upon the chromium content of metal adjacentto carbide particles, thereby impoverishing the metal in chromium belowthe concentra- Accordingly, a vulnerable pathway through the metal isopened up along the grain boundaries whenever carbide precipitationoccurs. When corrosion sets in along this pathway, the steel becomesunreliable for use under stress at either 'low or elevated temperatures.

The addition of one or more of the elements of the group consisting oftitanium and columbium to certain austenitic stainless steels preventsintergranular corrosion, where the additional elements are present insufficient quantity to tie up the carbon and thus arrest the formationof chromium carbides at the grain boundaries. In many of the steelswhich have either or both of the elements titanium and. columbiumpresent, though, for arresting intergranular corrosion, there stillremains a difficulty with sigma phase, and the steels accordingly aresusceptible to creep and stress'rupture at temperatures in theapproximate range of 1200 F. to 1500 R, these ingredients apparentlyintensifying the inclination to sigma phase formation. An appreciableloss of creep strength and stressto rupture strength occurs at thesetemperatures immediately with the formation of sigma phase. As "timeprogresses, deterioration of the high temperature providing stainlesssteel of the character indicated which 36 properties is progressive andaccelerated until eventual failure. Someembrittlement at roomtemperature also occurs after long exposure of the steel to roomconditions, though this perhaps is not too serious.

ticles of the steel which are strong and capable of withstanding sigmaphase development and the development of carbide precipitation andintergranular corrosion, thus to prevent rupture of the metal fromweakness or corrosion when in use.

Referring now more particularly to the practice of my invention, Iprovide austenitic stainless steel products and articles of the steel,which in including elements such as chromium and nickel in criticalamounts along with proper quantities of one or both of the elementstitanium and columbium, are for the intents and purposes free of sigmaphase and have excellent high temperature prop-- erties inclusive ofhigh resistance to creep and stressrupture and substantial freedom fromcarbide precipitation. More specifically, I provide stainless steelscontaining approximately 15% to 17% chromium, 9% to 12% nickel,manganese preferably ranging up to about 2.0%, one or both of the groupconsisting of titanium and columbium in minimum amount of (Ti+% Cb) :5times the carbon content and in maximum amount of (TH- A Cb)'=0 .8 0%and the remainder substan- Wherecolumbium is absent from the steel, itwill be understood that the titanium is present in minimum amount of 5times the carbon and in maximum amount of 0.80%. Where the elementtitanium is absent from the steel, columbium is present in minimumamount of 8 times the carbon and in maximum amount of 1.30%.

My stainless steels are substantially wholly austenitic in structure.Ferrite, if present at all, is only in traces. This I find is essentialto the required stress-rupture properties. Where appreciable amounts offerrite are present (say above about 2%) the'stress-rupture values falloff; also the working properties of the metal suffer.

The composition limits given are in fact considered to be critical inevery senseforj I find that where they are departedfrom one or more of'the desired qualities suffer. The chromium content of the steel where inexcess of about 17% too readily promotes sigma phase and where belowabout 15% is apt to be so small as to leave the steel susceptible toheat scaling. Nickel also is critically defined for when below about 9%,sigma formation is detrimental, and if the nickel is above approximately12% a loss in the creep and stress-rupture properties is experienced.

The carbon component of the steel not only is preferably restricted toamounts ranging up to about 0.10%, but is related'to the amount oftitanium and columbium or both when both are present. Should one or bothof the latter elements be used to excess, the excess will promote sigmaformation. Where the titanium and/or columbium are present 'inaccordance with the ranges indicated, however, the steel not only isstable with reto tie up the carbon and prevent carbide precipitation andimpoverishment of the chromium at the metal'grain boundaries. The steelsaccordingly are stable with regard to intergranular corrosion and thus"are not likely gard to sigma phase formation but these elements serve,

to develop weaknesses at the metal grain boundaries,

temperatures. Titanium, moreover, is a nitrogen inhibitor and thereforeis particularly useful should enough 'which will impair the loadcarrying capacity'at high I limit the quantity of silicon in my hightemperature steel to amounts ranging up to about 1% because this is acommercially practical and acceptable limit and even more importantlybecause excess silicon tends to promote sigma phase. ferred primarilyfor assuring hot workability of the steel and because of beingcommercially practical. In certain instances, however, 'I use largerquantities of manganese and adjust the chromium-nickel balance for anyexcess above 2%, this by regarding the excess manganese as a nickelsubstitute.

Among the products or articles which 'I make of the steel are chemicalapparatus and equipment for operating at elevated temperatures and underhigh pressures. Thus,

for example, I often provide containers as in the form ofvessels, tanks,tubing, orthe' like, of the steel for use in oil cracking units or inother chemical handling systems such'as where incandescent gases 'orother fiery chemicals are encountered. I find that the steel is amenableto welding'and to any of a host of other fabricating operations such ascutting and'punching, and therefore,'

I often resort to one or more of these operations in'producing certainproducts, as for example seam-welded tubes of the steel. In use, theproducts and articles are The manganese limit of 2% is prethoroughlycapable of withstanding the effects of high temperatures up to 1500 F.or more without hurtful sigma formation or carbide precipitation. Highpressure and elevatedtemperature operating conditions such as in thepressure fluid containers which I make of the steel are satisfactorilyaccommodated. In this, the metal reliably resists creep andstress-rupture and is not susceptible tof intergranular corrosion such.as might. lead to weaknesses and rupture under the temperatures andpressures.

Thus it will be seen that in this invention there are provide'd'hightemperature austenitic stainless steels and products and articles of thesteels in which the various objects noted, together with many thoroughlypractical advantages, are successfully achieved. It will be seen thatthe products are strong and durable and are well adapted to withstandhigh temperatures while under load even over long periods of time. Also,it will be appreciated that the steels are corrosion resistant in hotcorrosive atmospheres or when exposed to chemicals which corrodeordinary low-carbon steels. My products, it will be seen, have theremarkable ability to resist sigma phase development, and are all themore strong, durable and corrosion resistant in view of their stabilityagainst carbide precipitation and intergranular corrosion.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiment hereinbefore set forth, it is tobe understood that all matter described herein is to be interpreted asillustrative and not as a limitation.

' I claim:

1. High temperature duty austenitic stainless steel characterized bygood resistance to stress-rupture and to creep at 1200 to 1500 F. ormore containing 15% to 17% .chromium, 9% to 12% nickel, up to 1%silicon, carbon, metal of the group consisting of titanium and columbiumin minimum amount of (Ti+% Cb) :5 times the carbon content and inmaximum amount of (Ti+% Cb) 0.80%, and the remainder substantially alliron, said chromium, nickel, silicon, carbon, titanium and columbiumbeing present in such relative amounts that the steel is fullyaustenitic with ferrite content not exceeding 2% and free of sigma phaseand carbide precipitation at elevated temperatures.

2. High temperature duty austenitic stainless steel characterized bygood resistance to stress-rupture and to creep at 1200 to 1500 F. ormore containing 15% to 17% chromium, 9% to 12% nickel, up to 1% silicon,carbon, titanium from 5 times the carbon content up to 0.80%, and theremainder substantially all iron, said chromium, nickel, silicon, carbonand titanium being present. in such relative amounts that the steel isfully austenitic with ferrite content not exceeding 2% and free I ofsigma phase and carbide precipitation at elevated temperatures.

3. High temperature duty austenitic stainless steel characterized bygood resistance to stress-rupture and to creep at 1200 to 1500" F. ormore containing 15% to 17% chromium, 9%; to 12% nickel, up to 1%silicon,

carbon, columbium from 8. times the carbon content up to 1.30%, and theremainder substantially all iron, said chromium, nickel, silicon, carbonand columbium being present in such relative amounts that the steel isfully austenitic with ferrite content. not exceeding 2% and free ofsigma phase and carbide precipitation at elevated temperatures.

References Cited in the file of this patent

1. HIGH TEMPERATURE DUTY AUSTENITIC STAINLESS STEEL CHARACTERIZED BYGOOD RESISTANCE TO STRESS-RUPTURE AND TO CREEP AT 1200 TO 1500* F. ORMORE CONTAINING 15% TO 17% CHROMIUM, 9% TO 12% NICKEL, UP TO 1% SILICON,CARBON, METAL OF THE GROUP CONSISTING OF TITANIUM AND COLUMBIUM INMINIMUM AMOUNT OF (TI+5/8 CB)=5 TIMES THE CARBON CONTENT AND IN MAXIMUMAMOUNT OF (TI+8/13 CB=0.80%, AND THE REMAINDER SUBSTANTIALLY ALL IRON,SAID CHROMIUM, NICKEL, SILICON, CARBON, TITANIUM AND COLUMBIUM BEINGPRESENT IN SUCH RELATIVE AMOUNTS THAT THE STEEL IS FULLY AUSTENITIC WITHFERRITE CONTENT NOT EXCEEDING 2% AND FREE OF SIGMA PHASE AND CARBIDEPRECIPITATION AT ELEVATED TEMPERATURES.